                1. Field of the Invention        
The present invention relates to a circuit for biasing an input node of a sense amplifier.                2. Description of the Related Art        
Sense amplifiers are commonly used in several applications (such as for reading cells of a non-volatile memory device). A sense amplifier consists of a circuit that detects a low-level signal applied to an input terminal thereof; typically, the sense amplifier compares the input signal with a reference signal, and outputs two different values according to the result of the comparison.
Operation of the sense amplifier requires that its input terminal be kept at a pre-set voltage. For example, in a non-volatile memory device (wherein each memory cell consists of a floating gate MOS transistor) the sense amplifier provides the correct biasing of the drain terminal of a selected memory cell to be read. The biasing voltage is set to a value (such as 1 V), which is a compromise between the opposed requirements of having a low biasing voltage (for avoiding any undesired writing of the selected memory cell) and a high biasing voltage (for having the input signal at a level sufficient to enable the reading operation).
Correct and accurate biasing of the drain terminal of the selected memory cell is of the utmost importance. In fact, any increase of the biasing voltage may result in the spurious writing of wrong values in the memory device; on the other end, any decrease of the biasing voltage may result in the reading of wrong values.
This problem has been exacerbated in the last years by the use of technological processes that have brought about a substantial reduction in the gain of the transistors implementing the memory cells. Moreover, the problem is particular acute in multilevel memory devices, wherein the distance between the distributions of the input signal associated with two adjacent values stored in each memory cell is strongly reduced (especially when a very low power supply voltage is used).
In order to keep the input node at the desired biasing voltage with a high degree of accuracy, most of the sense amplifiers known in the art include a voltage regulator; typically, the voltage regulator consists of a transistor in a cascode configuration that is controlled by a feedback inverter.
Document U.S. Pat. No. 6,320,790, which is assigned to the assignee of the present application and is incorporated by reference herein in its entirety, also proposes the use of a constant current generator for biasing the feedback inverter; the generator provides a current that is independent of the temperature and of the power supply voltage. This solution reduces the spread of the threshold voltage of the inverter, thereby increasing the accuracy of the feedback control on the voltage at the input node.
However, the high accuracy of the structures described above adversely affects the dynamic response of the sense amplifier, and then the performance of the whole memory device. Moreover, this drawback prevents the solutions known in the art to be used in high performance devices (for example, with an access time lower than a few tens of ns).